In recent years, a semiconductor device with low power consumption has started to be used as a component in an electronic device for a reduction in the power consumption of the electronic device. An electronic device includes a variety of circuits such as a CPU, an interface circuit, and a memory element. These circuits are connected by an input circuit, an output circuit, or an input/output circuit.
As an input circuit, an output circuit, and an input/output circuit, a buffer circuit or a three-state inverter (also referred to as “tri-state inverter”) circuit is used. In the three-state inverter circuit, output is set to the following three states: “High (HI),” “Low (LO),” and “High impedance (HIZ).”
“High” of the three-state inverter circuit means that the potential of an output terminal is set to the highest potential of the power supply voltage terminal; “Low” thereof means that the potential of the output terminal is set to the lowest potential; and “High impedance” thereof means that the output terminal is set in a floating state.
Transistors included in a three-state inverter each have leakage current, so that leakage of charges or inflow of charges occurs even when the transistor is not selected. Thus, even when the three-state inverter is brought into a high impedance state, leakage current flows to an output terminal through the transistor included in the three-state inverter, which results in an insufficient reduction in power consumption. Further, voltage drop occurs, which causes malfunction.
Patent Document 1 discloses a flip-flop circuit using a three-state inverter in which a transistor with high threshold voltage and a transistor with low threshold voltage are provided together to reduce leakage current flowing when the flip-flip circuit does not operate.
However, in Patent Document 1, high power supply voltage is needed in consideration of the transistor with high threshold voltage in order to secure the operation of the transistor with high threshold voltage. Further, the transistors are each thought to be a transistor formed using a silicon material, and leakage current is kept flowing in the transistors even when the transistors are turned off. Thus, a sufficient reduction in power consumption is not achieved.